Gears, pulleys, and various other power transmission members formed from engineering plastics offer certain advantages over their metal counterparts, including lower weight and potentially lower manufacturing and material costs. As is known in the prior art, the manufacturing of plastic gears and pulleys can be simplified by integrally forming a shaft by which the gear or pulley is rotatably supported and secured to an appropriate frame, housing or any other suitable support structure. Shafts for idler wheels, which generally include idler gears, sprockets and pulleys, are often formed to include a feature which will retain the wheel to its support structure, yielding an idler wheel assembly which can be readily manufactured and assembled in mass production.
Illustrated in FIG. 1 is an example of such an idler gear assembly 110. The assembly 110 is composed of an idler gear 112 mounted to a support frame 114 by an integrally-formed shaft 116 received in an appropriately sized bore 118 in the support frame 114. As shown, the shaft 116 extends from one surface of the idler gear 112 and defines the gear's axis of rotation. In order to allow the idler gear 112 to rotate freely, the support frame 114 includes a boss 120 which circumscribes the bore 118 so as to space the idler gear 112 from the facing surface 122 of the support frame 114. The shaft 116 is retained in the bore 118 by a pair of barbs 124 which engage the rear surface 126 of the support frame 114. If access to the rear surface 126 is prevented by the configuration of the support frame 114, the mounting of the idler gear 112 is rendered substantially tamper-proof in that the barbs 124 prevent the shaft 116 from being removed from the bore 118.
The above structure is generally known in the prior art, and yields an idler gear 112 which can be readily assembled with its support frame 114 by simply aligning the shaft 116 with the bore 118 and forcing the barbs 124 through the bore 118 until they are allowed to resiliently re-expand at the rear surface 126 of the support frame 114. However, the above structure does have certain disadvantages, including those which adversely effect the reliability and manufacturability of all such idler wheels. One critical aspect is that, during assembly, there is the possibility that the shaft 116 may not be sufficiently assembled with the support frame 112 in order to deploy the barbs 124 against the rear surface 126 of the support frame 114. Such circumstances are particularly possible if the interference between the barbs 124 and the bore 118 necessitates an assembly force which exceeds an anticipated upper process limit or capability. Such a possibility requires in-process inspections, which complicate processing and increase manufacturing costs. Due to the interference fit between the barbs 124 and the bore 118 of an incompletely assembled assembly 110, the idler gear 112 may appear properly secured to the support frame 114. However, once placed in service, the shaft 116 will eventually become dislodged from the bore 118.
Another disadvantage is that the molding of the idler gear 112 is complicated by the requirement for the barbs 124 to resiliently withstand significant compression during the assembly process as they travel the full length of the bore 118. Such a requirement necessitates certain design tradeoffs between resiliency and strength of the barbs 124 under certain conditions. Furthermore, depending on the relative sizes of the bore 118 and the effective diameter of the barbs 124, the force required to assemble the idler gear 112 with the support frame 114 may be excessive, resulting in unacceptable damage to the idler gear 112 and/or the support frame 114.
Thus, it would be desirable to provide a relatively low cost idler wheel assembly which can be readily assembled in mass production, and is configured to enable its reliability to be easily and dependably ascertained immediately after assembly. In addition, such an idler wheel assembly would require a minimal assembly force in order to mount the idler wheel to its support member, and also provide for enhanced moldability of the idler wheel and its shaft.